Long Term Evolution, LTE, is the fourth-generation, 4G, mobile communication technologies standard developed within the 3rd Generation Partnership Project, 3GPP, to improve the Universal Mobile Telecommunication System, UMTS, standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, and lowered costs. The Universal Terrestrial Radio Access Network, UTRAN, is the radio access network of a UMTS and Evolved UTRAN, E-UTRAN, is the radio access network of an LTE system. In a UTRAN and an E-UTRAN, a User Equipment, UE, is wirelessly connected to a Radio Base Station, RBS, commonly referred to as a Node B, NB, in UMTS, and as an evolved Node B, eNodeB or eNodeB, in LTE. An RBS or access node is a general term for a radio network node capable of transmitting radio signals to a user equipment, UE, and receiving signals transmitted by a user equipment.
Currently, mobile communication technologies are evolving to very high frequency, larger carrier bandwidth, very high data rate and multiple heterogeneous layers. The future mobile network, also called 5G mobile network, is likely to be a combination of evolved 3G technologies, 4G technologies and emerging or substantially new components such as Ultra-Density Network, UDN. Due to the increasing demand to enhance wireless capacity and due to lack of availability of spectrum in lower frequency range, e.g. 800 MHz-3 GHz, the use of frequencies in 10's of GHz range are being investigated. Investigations are going on to explore the high frequency bands, for instance, in the range of 30 GHz, 60 GHz and 98 GHz for future mobile networks also known as 5G networks. At this frequency very large spectrum is available. This means both operating frequency and bandwidth for 5G networks are expected to be much higher than that used in the legacy mobile network e.g. 3G and 4G networks. However, due to large signal attenuation with respect to path loss, the network operating over such high frequencies is supposed to cover hotspot areas with densely deployed radio access nodes or base stations. Such dense deployment provides sufficient coverage for indoor called “hot areas”.
FIG. 1a illustrates a heterogeneous network. The macro cell covers a large area. Within the area by the macro cell, there are a number of UDN network nodes, e.g. UDN node 1 (UDN1) and UDN node 2 (UDN2), and one micro cell.
In principle macro cells are supposed to cover most of area and micro/pico cell are supposed to boost the capacity and compensate the possible coverage holes within the macro cells. The UDN networks over very high frequencies are supposed to boost the capacity. The UDN network, macro cell and pico cells may also operate over different frequencies. A frequency is also interchangeably called as carrier, layer, frequency layer, channel or carrier frequency.
In existing mobile communication systems like in LTE the typical channel bandwidth varies between 5-20 MHz. But in in future systems like in UDN the channel bandwidth will be several times larger than in LTE in order to meet very high data rate requirements. Very large BW is achievable due to the availability of very large spectrum in very high frequency range. Frequencies in the range 30 to 300 gigahertz are often referred to as Extremely high frequency, EHF, Radio waves in this frequency range have wavelengths from about ten to around one millimeter, giving it the name millimeter band or millimeter wave, sometimes abbreviated MMW or mmW.
However the introduction of UDN will also significantly increase the UE power consumption due to 1) much large carrier bandwidth of millimeter wave, 2) large possible spectrum of UDN and 3) indoor/hot area deployment policy of UDN. The UE has to regularly measure on multiple cells on the serving and non-serving carrier frequencies for the purpose of mobility. Due to very dense deployment in UDN, the measurements also have to be done over very large number of cells. Therefore in UDN the measurements on larger BW will lead to significant increase in the UE power consumption. The existing mechanisms for cell detection/measurement are therefore not adequate for UDN operation with dense deployment and over very large BW.
The UE power consumption due to both cell detection and measurements of detected cells seriously impact the battery life. Therefore in LTE for both idle and connected states the Discontinuous Reception, DRX, cycle is used. The use of DRX cycle is one of the most viable methods to save UE battery life. In order to further enhance UE battery life, i.e. lower power consumption, there also exist solutions for further reducing power consumption in e.g. UDN networks. There are both network controlled methods and UE based methods as exemplified and explained below.
Existing Network Controlled UE Power Saving Methods
Patent application WO2013137700A1 discloses a method for reducing consumption of battery power of User Equipment, UE, during inter-frequency cell detection in a Heterogeneous Network with the assistance from the macro cell. The UE only starts to monitor the beacons, i.e. pilots, of certain small cells when the macro network indicates there are active small cells. Patent application US20120322440A1 discloses methods to limit the number of cells to measure for low mobility UEs so that the UE reception power can be saved via configuration with system information.
Existing UE Controlled Power Saving Methods
The UE based methods relies on UE to adapt its carrier monitoring. As a first example, a UE may stop monitoring other carriers when the radio quality of the present camped carrier is good enough. As a second example, a UE may decrease the measurement rate gradually for certain carrier or RAT.
The UE power saving method in this case is based on the assumptions of UE vendors i.e. UE implementation specific. The cell detection/measurement performance (accuracy and measurement delay requirements) may not be guaranteed.
Due to the above factor the UE power consumption and complexity for cell detection and measurement in UDN is expected to be much higher than in the current networks. The existing methods therefore cannot provide good enough power saving and/or guarantee the detection/measurement delay and/or accuracy.
Since the UDN are supposed to provide indoor coverage and hot area coverage, it will be difficult for the macro cell, e.g. LTE, to accurately determine the covered area of UDN even if the macro cell knows the deployment of UDN networks. When the macro cell informs the UE the existence of UDN coverage according to WO2013137700A1, a UE has to keep monitoring UDN cells in the area covered by the macro cell even though the UDN coverage is a small compared to the macro cell coverage, see FIG. 1a. In big cities where UDN network covers hotspot/indoor area, the power saving effect for UDN cell detection of this method can be clearly decreased. The method to limit the number of cells as disclosed in US20120322440A1 may then not be able to detect the strongest UDN cells in time.
Based on the above analysis, a more advanced power saving method to ensure the cell detection/measurement delay/accuracy for heterogeneous network operating on low frequency band and millimeter wave band is desired.